Metallic components, such as turbine blades in a gas turbine engine, commonly include a thermal barrier coating which protects the components from hot gases present in the engine. The thermal barrier coating is typically thin, i.e., on the order of 5 to 9 thousandths of an inch. The barrier coating is typically formed as a ceramic-like material having a low heat transfer coefficient and having a high melting point. For example, yttria stabilized zirconia is often used as a thermal barrier protective coating on gas turbine hot section components. The coating is typically applied by either thermal spray or vapor phase techniques. The thickness of the coating is used to control the temperature gradient developed between the surface of the component and the base material to which the coating is applied.
Due to process variations, it is difficult to determine the thickness of a coating applied to complex geometry components on a part to part basis. Generally, destructive evaluation is used to evaluate the coating thickness during an initial set-up of the coating process, followed by a combination of non-destructive measurement and sacrificial test pieces used for ongoing process control of the coating thickness. For example, eddy current thickness measurement is a widely used non-destructive in-process control tool to verify the thermal barrier coating thickness. However, an issue associated existing non-destructive methods, such as eddy current testing, is that it is difficult to define the location of the interface between the coating and the base material, especially in highly curved areas of the component, or where the base material is porous or includes significant defects.
Infrared flash tomography is often used with thermal barrier coatings to identify areas of delamination, spalling or defects that may occur in the coating or at the interface between the coating and the base material. Infrared tomography heats the thermal barrier coating at a known rate and measures the reflected heat signature emitted from the test piece, where it is believed that a measurement of the heat decay with time can provide a value indicative of the coating thickness. In addition, mid-infrared reflectance has been used to inspect the health of a thermal barrier coating using changes or variations of the reflectance of light from the coating, as determined relative to a known standard, to determine a level of internal damage to the coating layer. Optical coherence tomography has also been applied as a method of determining the coating thickness, but requires calibration of the system against reference samples having a known thickness.
There continues to be a need to provide a non-destructive inspection method for determining the thickness of a thermal barrier coating that may be implemented during a manufacturing process for applying the coating, as well as at other times such as during routine inspection of components.